The present invention is concerned with mercury cadmium telluride semiconductor devices. In particular, the present invention is directed to the preparation of reliable ohmic contacts to p-type mercury cadmium telluride. For the purposes of this specification, the common chemical equations for mercury cadmium telluride, (Hg,Cd) Te or Hg.sub.1-x Cd.sub.x Te, will be used.
Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a wide gap semiconductor (E.sub.g =1.6 eV), with mercury telluride, which is a semi-metal having a negative energy gap of about -0.3 eV. The energy gap of the alloy varies approximately linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting x, it is possible to obtain (Hg,Cd)Te detector material having a peak response over a wide range of infrared wavelengths. High performance (Hg,Cd)Te detectors have been achieved for wavelengths from about 1 to 30 microns.
In the prior art, techniques have been developed for making ohmic contact to p-type (Hg,Cd)Te. For example, one technique involves the deposition of gold on a p-type (Hg,Cd)Te surface. The gold and the (Hg,Cd)Te are then heated to cause diffusion of the gold into the (Hg,Cd)Te. In this manner, a higher conductivity p-type region (a "p+ region") is formed near the gold contact. Electrical leads are then attached to the gold with a bonding material such as an indium solder.
The prior art techniques, however, have not been completely satisfactory in making reliable ohmic contacts to p-type (Hg,Cd)Te. In particular, the prior art contacting techniques have proved to be unsatisfactory at very low temperatures (for example, less than about 50.degree. K). At these low temperatures, the prior art contacts tend to exhibit non-ohmic behavior and excess noise.
Because of the vapor pressures of the constituents of (Hg,Cd)Te, the material cannot be heated above about 700.degree. K for any extended period without changing the electrical properties. Any successful contacting technique for (Hg,Cd)Te, therefore, must be capable of being performed at less than about 700.degree. K. This limits the bonding materials which may be used to materials having a low melting point.